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Bureau of Mines Information Circular/1983 



Agglomeration-Heap Leaching 
Operations in the Precious 
Metals industry 

By G. E. McClelland, D. L. Pool, and J. A. Eisele 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8945 



Agglomeration-Heap Leaching 
Operations in the Precious 
Metals Industry 

By G. E. McClelland, D. L. Pool, and J. A. Eisele 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 



Library of Congress Cataloging in Publication Data: 



Xt^ 









McClelland, G. E 








Agglomeration-heap 


eaching operations in the 


precious metals 


industry. 


» 






(Information circular 


/ United States 


Department of the Interior, 


Bureau of Mines ; 8945) 








Bibliography: p. l6. 








Supt. of Docs, no.: 1 


28.27:8945. 






1. Precious metals 


—Metallurgy. 2. 


Agglomeration. 3. Spoil 


banks— Leaching. I. Poc 


1, D. L. (Danny 1 


..). II. Eisele, J. A. (Judith 


A.). III. Title. IV. Scries: Information 


circular (U 


nited States. Bu- 


reau of Mines) ; 8945. 








-T-?f30^vU4- [TN759] 


622s [669'. 


22'028] 


83-600179 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Agglomeration concept 2 

Commercial agglomeration-heap leaching operations 3 

Arizona gold heap leaching , 20 tpd 3 

Colorado gold vat leaching , 1 , 500 tpd 4 

Arizona silver heap leaching, 2,000 tpd 6 

Northern Nevada gold heap leaching , 2 , 500 tpd 7 

Eastern Nevada gold heap leaching, 3,000 tpd 10 

Summary and conclus ions 14 

References 16 

ILLUSTRATIONS 

1 . Small Arizona gold agglomeration-heap leaching operation 3 

2. Discharge end of the drum agglomerator used at the Colorado gold 

agglomeration-vat leaching operation 5 

3. Leaching vat at the Colorado gold agglomeration-vat leaching operation.... 6 

4. Agglomeration at the Arizona silver heap leaching operation showing the 

reverse belt agglomerator 8 

5. Agglomerated ore stockpile at the Arizona silver heap leaching operation.. 8 

6. Agglomerated heaps during leaching at the Arizona silver operation 9 

7. Gold agglomeration-heap leaching operation in northern Nevada 10 

8. Heap leaching gold ore in northern Nevada before agglomeration pretreat- 

ment was adopted 11 

9. Agglomerated gold ore being heap leached in northern Nevada 11 

10. Overall view of the eastern Nevada agglomeration-heap leaching operation. . 12 

11. Drum agglomerator used in the eastern Nevada operation 12 

12. Agglomerated heaps being leached in the eastern Nevada gold heap leaching 

operation 13 

TABLE 

1. Summary of agglomerating conditions of the five commercial operations 

described 15 





UNIT OF MEASURE ABBREVIATIONS USED 


IN THIS REPORT 


°c 


degree Celsius 


pet 


percent 


ft 


foot 


psig 


pound per square inch, 
gauge pressure 


gal 


gallon 










rpm 


revolution per minute 


gpm 


gallon per minute 










sec 


second 


gpm/ft^ 


gallon per minute 








per square foot 


tpd 


ton avoirdupois per day 


hr 


hour 


tph 


ton avoirdupois per hour 


in 


inch 


tr oz 


troy ounce 


lb 


pound 






lb/ton 


pound per ton 
avoirdupois 







AGGLOMERATION^HEAP LEACHING OPERATIONS 
IN THE PRECIOUS METALS INDUSTRY 

By G. E. McClelland/ D. L. Pool,^ and J. A. Eiseie^ 



ABSTRACT 

During the 1970' s, the Bureau of Mines investigated a particle ag- 
glomeration technique for improving the flow of leaching solution 
through heaps of clayey or crushed, low-grade gold-silver ores, wastes, 
and tailings. This technology has been adopted on a broad scale by the 
precious-metals-processing industry. This report presents information 
on five commercial operations that have benefited from agglomeration 
technology and that represent a cross section of current heap leaching 
practice. The technology is cost effective because of decreased leach 
times and improved precious metal recoveries. 



^Metallurgist. 
^Research chemist. 
-'Supervisory chemical engineer. 
All authors are with the Reno Research Center, Bureau of Mines, Reno, Nev. 



INTRODUCTION 



Exploration during the mid to late 
1970's identified numerous low-grade pre- 
cious metal deposits, mine waste mate- 
rials, and tailings piles throughout the 
Western United States. The increase in 
precious metal prices during this period 
generated interest in processing these 
low-grade feed materials by low-cost heap 
leaching technology. Heap leaching with 
cyanide was applied to many of the mate- 
rials; however, many conventional oper- 
ations were unsuccessful because exces- 
sive amounts of clay in the feed or fines 
generated during crushing prevented a 
uniform flow of cyanide solution through 
the heaps. 

As part of its research program to im- 
prove the recovery of gold and silver 
from low-grade domestic resources, the 
Bureau of Mines investigated agglomera- 
tion pretreatment to overcome the perco- 
lation problems associated with heap 
leaching. Agglomeration of the clays and 
fines before heap building permitted 
a uniform and rapid flow of cyanide 



leaching solution through the heaps. 
Previous Bureau publications (l.~2^)^ de- 
scribe bench- and pilot-scale experiments 
showing the advantages of particle ag- 
glomeration as a pretreatment for mate- 
rials that are difficult to treat by 
standard heap leaching techniques. 

This report shows how the Bureau's re- 
search on agglomeration-heap leaching has 
been adopted and applied by the precious- 
metals-processing industry. It describes 
five commercial operations, ranging in 
size from 20 to 3,000 tpd and represent- 
ing a cross section of current heap 
leaching practice, that have benefited 
from agglomeration technology. The prob- 
lems they experienced in trying to apply 
conventional heap leaching to ores con- 
taining clay ranged from complete plug- 
ging of the heap, with no solution flow, 
to slow solution flow, with long leach- 
ing times for precious metal recovery. 
Agglomeration pretreatment effectively 
resolved the heap permeability problems 
encountered by these operators. 



AGGLOMERATION CONCEPT 



Most precious metal ores require crush- 
ing to minus 1 in or finer before agglom- 
eration. Crushing to these sizes liber- 
ates precious metal values and improves 
overall recovery. Crushed ores can be 
agglomerated by mixing 5 to 10 lb port- 
land cement per ton of dry feed, wetting 
with 8 to 16 pet moisture as either water 
or strong cyanide solution, mechanically 
tumbling the wetted mixture, and curing 
the agglomerated feed for a minimum of 8 
hr before heap leaching. The quantity of 
cement added during agglomeration usually 
provides the protective alkalinity re- 
quired for cyanide leaching. After ag- 
glomeration and heap building, leaching 
is conducted with conventional heap 
leaching techniques. 

During agglomeration, the clay and fine 
particles contained in the ore adhere 
to the coarser particles and create a 



coating of fines around the coarse parti- 
cles. The agglomerates produced are of 
sufficient green strength after curing to 
withstand wetting with minimal degrada- 
tion. Agglomeration overcomes the major 
problems associated with particle segre- 
gation during heap building O) , fines 
migration, and solution channeling during 
leaching by producing a porous, permeable 
feed (_4). 

A permeable feed material stacked in a 
heap permits the uniform flow of leaching 
solution and contact of the cyanide 
leaching solution with the exposed pre- 
cious metal particles and decreases the 
leaching time required to obtain targeted 
precious metal recovery, 

'^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



COMMERCIAL AGGLOMERATION-HEAP LEACHING OPERATIONS 



ARIZONA GOLD HEAP LEACHING, 20 TPD 

A small Arizona operation (fig. 1) pro- 
cessing 20 tpd of gold ore changed to ag- 
glomeration technology after conventional 
heap leaching was unsuccessful because of 
inadequate leaching solution percolation. 

The oxidized disseminated deposit con- 
tains an average 0.2 tr oz of gold per 
ton. The host rock is porous and the 
gold particles are exposed. The ore is 
mined from an open pit, transported to 
the crushing plant by a front-end loader, 
crushed to a nominal 3/8-in feed size, 
and conveyed to a drum agglomerator. 

The drum agglomerator is 3 ft in diame- 
ter and 7 ft in length. A static scrap- 
er, located lengthwise in the top portion 
of the drum, prevents excessive buildup 



of clayey material. A spray bar adjacent 
to the scraper emits a spray that covers 
a 5-ft length of the drum. The drum 
slopes 5° from feed to discharge end and 
rotates at 17 rpm. Feed retention time 
is 20 sec. The drum can agglomerate 10 
tph of dry feed. 

The minus 3/8-in feed material is mixed 
with 10 lb Portland cement (type II) per 
ton of dry feed before discharge from the 
feed conveyor, and the mixture is blended 
in the first foot of the drum. The ore- 
binder mixture is wetted by the spray 
bar with a solution containing 2 lb NaCN 
per ton. Total moisture required for ag- 
glomeration is 10 pet. On discharge from 
the drum, the agglomerated ore is con- 
veyed to an impervious leaching pad and 
stacked into a 7-ft-high heap that con- 
tains 100 tons. The agglomerated ore is 




FIGURE 1. - Small Arizona gold agglomeration-heap leaching operation. Shown are the open pit 
mine, the crushing circuit, the agglomeration equipment, and a heap being leached. 



cured in the heap for 48 hr before 
leaching. 

The heap is leached in 5 days by spray- 
ing solution containing 1 lb NaCN per ton 
over the heap at a rate of 0.033 gpm/ft^. 
The cement added during agglomeration is 
sufficient for protective alkalinity. 
The dissolved gold is recovered from the 
pregnant solution by passing the solution 
upward through four 14-in by 5-ft carbon 
columns at a rate of 20 gpm/ft^. The 
barren solution is recycled to the heap. 

The loaded carbon is stripped with an 
alkaline-alcohol solution. The precious 
metal values are electrowon from solution 
on steel wool cathodes in a cylindrical 
electrowinning cell. The gold-laden 
cathodes are treated with nitric acid to 
dissolve excess iron, and the resultant 
sludge is fire refined to produce a dore 
bullion. 

Gold recoveries obtained by agglomera- 
tion and heap leaching average 90 pet. 
Agglomeration pretreatment improved heap 
permeability, percolation rate, gold re- 
covery, and decreased the leaching period 
required to obtain the targeted recovery. 

COLORADO GOLD VAT LEACHING, 1,500 TPD 

In this operation, the agglomerated 
feed is leached in large vats rather than 
in heaps. The operation is located in 
the Colorado Rocky Mountains southwest of 
Denver at an elevation of 10,000 ft (_5 ) . 

The gold ore occurs in an oxidized dis- 
seminated deposit and averages 0.06 tr oz 
of gold per ton with trace amounts of 
silver. The host rock is porous and the 
gold is liberated by crushing. The ore 
is mined from an open pit and is friable. 
Seventy-five percent of the deposit can 
be mined with dozers and front-end load- 
ers. The remaining 25 pet must be 
drilled and blasted. The operation mines 
and processes 1,500 tons of ore per day. 
All the material mined is ore; there is 
no waste rock. 



The mined ore is transported by front- 
end loader to the in-pit crushing plant 
where it is crushed to a 1/2-in size by 
two-stage crushing. The minus 1/2-in 
feed is conveyed uphill one-fourth mile 
to the processing plant. The ore trans- 
port conveyor system is cheaper to oper- 
ate than truck transport, which keeps the 
mining costs extremely low. 

The transport conveyor discharges the 
crushed ore onto a surge pile outside the 
agglomeration-vat leaching building. The 
enclosed building permits year-round 
operation. From the surge pile the ore 
is fed to an 8-1/2- by 32-ft rotating 
drum (fig. 2). The drum slopes 8°, with 
the feed end elevated, and rotates at 10 
rpm. The walls of the drum are not 
scraped because the agglomerated feed 
does not build up excessively. 

The ore is mixed with 10 lb of binder 
(3 lb Portland cement (type II), 7 lb 
fly ash) per ton of feed in the first 
few feet of the drum. The material is 
sprayed with a pH 12 solution containing 
10 lb NaCN per ton. A 20-ft spray bar 
inside the driom is used to increase the 
moisture content of the feed to 13 pet. 
Mechanical tumbling occurs along the 
length of the drum. The agglomerated 
feed is conveyed to a stockpile from 
which the leaching vats are loaded by a 
front-end loader. Curing occurs while 
the vats are being loaded. 

The feed is placed into one of four 
inclined vats measuring 80 by 50 ft. The 
deepest portion of the vat is 9 ft. Each 
vat contains 1,000 tons of agglomerated 
feed and requires 4 hr to load. 

The ore is leached by spraying solution 
at the rate of 0.05 gpm/ft^ until the vat 
is full. The solution in the vat is cir- 
culated and sprayed by pumps throughout 
the leaching cycle to insure an adequate 
dissolved oxygen content (fig. 3). 

Gold recovery is 90 pet in a 3-day 
leaching and washing cycle. The vats 



drain in approximately 8 hr. The mois- 
ture retained by the leached residue is 
13 pet, the same amount of moisture re- 
quired for agglomeration, so there is no 
entrained gold-bearing solution in the 
residue. The plant capacity was designed 
for 1,000 tpd, but changing the leaching- 
washing cycle from 4 to 3 days allows 



1,500 tons of agglomerated ore a day to 
be processed. 

The pregnant solution from the vats is 
pumped to a carbon adsorption system for 
gold recovery. The carbon system con- 
sists of four adsorption tanks, each con- 
taining 1.5 tons of 6- by 12-mesh coconut 




FIGURE 2. - Discharge end of the drum agglomerator used at the Colorado 
gold agglomeration-vat leaching operation. Drum diameter is 8=1/2 ft. 






FIGURE 3. = Leaching vat at the Colorado gold agglomeration=vat leaching operation. After flood- 
ing the vat, the solution is circulated through the agglomerated ore by pumping and spraying to main- 
tain an adequate dissolved oxygen content throughout the leaching period. 



shell activated carbon. When the lead 
carbon is loaded to 200 tr oz of gold per 
ton, it is educted to the desorption cir- 
cuit. Fresh carbon is kept in the last 
tank in the solution flow sequence to in- 
sure a good barren solution. The barren 
solution is pumped to a holding tank for 
recycle to the leaching vats. All rea- 
gent makeup is done during agglomeration. 

The loaded carbon is desorbed in 8 hr 
by caustic cyanide solution at 116° C and 
25 psig pressure. The pregnant solution 
from desorption is passed through one of 
four cylindrical electrowinning cells 
where the gold is deposited on steel wool 
cathodes. The cathodes are directly fire 



refined to produce dore bullion contain- 
ing 80 pet gold and 17 pet silver. 

Preliminary metallurgical testing had 
shown conventional heap leaching to be 
unsatisfactory. Agglomeration increased 
permeability, percolation rate, and gold 
recovery. Agglomeration permits a very 
short leaching cycle, which allows for 
increased gold production. 

ARIZONA SILVER HEAP LEACHING, 2,000 TPD 

A silver agglomeration-heap leaching 
operation is located near Tombstone, 
Ariz. (6). Two types of materials are 
mined: old waste material that was used 



for mine backfill and virgin ore adjacent 
to the waste material. The waste mate- 
rial is mined with front-end loaders to 
expose the virgin ore. The virgin ore is 
drilled and blasted and moved by front- 
end loader. The silver content of the 
two feed materials varies. The cutoff 
grade of feed to the heaps is 1.0 tr oz 
of silver per ton. Approximately 2,000 
tpd of ore is mined and agglomerated. 

The mined ore and waste are moved from 
a stockpile to the crushing plant where 
they are crushed to a nominal 1/2 in. 
Lime (7 lb/ton of ore) is used as the 
binder for agglomeration and is mixed 
with the ore during secondary crushing. 
The crushed ore-lime mixture is conveyed 
to an underground ore stockpile. Some 
moisture is sprayed onto the ore on the 
crusher discharge conveyor to decrease 
dusting. 

The ore from the underground stockpile 
is agglomerated on a reverse belt con- 
veyor designed by the operators (fig. 4), 
The 4- by 25-ft belt agglomerates ore at 
a rate of 200 tph. The agglomerating 
conveyor can be set at an angle between 
35° and 45° , and the belt travels upward 
while the ore moves down the belt. The 
angle and speed of the belt can be varied 
to provide the desired retention time of 
ore on the belt. Water is sprayed at 
several locations along the length of the 
belt and gives the agglomerated feed a 
moisture content of between 10 and 12 
pet. A small amount of moistened fines 
adheres to the belt and rides up the con- 
veyor. A scraper was placed on the bot- 
tom side of the drive roller to eliminate 
excessive fines buildup on the belt. 

The agglomerated ore is transported to 
a stockpile by a radial arm stacker (fig. 
5), The material from the stockpile is 
trucked 500 ft to a 3/4-acre impervious 
leaching pad and is allowed to cure dur- 
ing heap building. Five heaps, each con- 
taining 6,000 tons of agglomerated ore 
stacked 10 to 11 ft high, are leached on 
the pad (fig, 6). Three heaps are at 
different stages of leaching, while the 
other two are either being prepared for 
leaching or being removed from the pad. 



The heaps are sprayed with pH 10,5 so- 
lution containing 2 lb NaCN per ton at a 
rate of 0,0075 gpm/ft^. The leaching so- 
lution percolates through the heap, is 
collected on the impervious leaching pad, 
and drains into plastic-lined solution 
trenches , The leaching and washing cycle 
is 7 days. The leached residue is trans- 
ferred to an auxiliary leaching pad 
and sprayed with cyanide solution 1 day 
per month for additional precious metal 
extraction. 

Precious metal values in the pregnant 
solution are recovered by Merrill-Crowe 
zinc precipitation technology. The 
precious-metal-bearing zinc precipitates 
are refined on site and yield dore bul- 
lion. The dore'' is shipped to another 
facility for refining. The barren solu- 
tions are recycled to the heaps. 

Heap leaching was unsuccessful before 
agglomeration pretreatment was applied to 
the ore. Conventional heap leaching re- 
covered only 37 pet of the leachable sil- 
ver from 3-in feed material treated in 
90-day leaching cycles. Severe percola- 
tion problems were encountered. Agglom- 
eration permitted finer crushing, which 
liberated additional silver values for 
dissolution by cyanide. Agglomeration 
heap leaching increased silver recovery 
to 90 pet of the leachable silver and de- 
creased the leaching time to 7 days, 

NORTHERN NEVADA GOLD HEAP 
LEACHING, 2,500 TPD 

An operation in northern Nevada (fig, 
7) , which has produced gold by agitation 
cyanidation and countercurrent decanta- 
tion for several years , discovered a new 
ore deposit several miles from the work- 
ing mine. Higher grade ore from the new 
deposit is transported to the original 
mill for gold recovery. Ore containing 
less than 0.07 tr oz gold per ton is heap 
leached at the site. Average ore grade 
for the heap material is 0.034 tr oz gold 
per ton. The ore is mined by open pit 
methods and is trucked to the heap leach- 
ing site approximately one-half mile from 
the new pit. About 2,500 tpd of ore is 
mined and heap leached. 




FIGURE 4. - Agglomeration at the Arizona silver heap leaching operation showing the reverse belt 
agglomerator. 



FIGURE 5. - Agglomerated ore stockpile at the Arizona silver heap leaching operation. Regions of 
fine material in the right portion of the photograph indicate insufficient moisture was added during 
agglomeration. 




FIGURE 6. - Agglomerated heaps during leaching at the Arizona silver operation. In the foreground 
is agglomerated ore after leaching. 



The ore is crushed to minus 5/8 in by a 
primary jaw crusher and a secondary cone 
crusher. Portland cement (type II), at 
the rate of 7 to 10 lb/ton of ore, is 
added to the ore at the jaw crusher dis- 
charge conveyor and is mixed with the 
ore during secondary crushing. The ore- 
binder mixture from secondary crushing is 
conveyed to a radial arm stacker. Water 
is sprayed onto the mixture at the dis- 
charge end of the stacker and gives a 
final moisture content of 9 to 13 pet. 
The ore is agglomerated by cascading down 
the sides of the conical-shaped agglom- 
erated ore stockpile. Additional tum- 
bling, sufficient to effect agglomera- 
tion, occurs when the front-end loader 
loads the rear dump truck with agglom- 
erates from the stockpile and when the 



truck dumps the agglomerated feed onto 
the impervious asphalt leaching pad. The 
agglomerated ore is cured for 2 to 3 days 
while the heap is being built. 

Five 17,000-ton heaps, approximately 12 
ft high, are built on the leaching pad. 
Three heaps are in different stages of 
the leaching cycle while the remaining 
two are either being prepared for leach- 
ing or being removed from the pad. 

The agglomerated heaps are leached by 
spraying 0.004 to 0.005 gpm/ft^ of pH 
10-11 solution containing 1.0 lb NaCN per 
ton of solution. The portland cement 
added during agglomeration provides most 
of the alkalinity during leaching, but 
small quantities of NaOH are added to the 



10 




FIGURE 7. = Gold agglomeration-heap leaching operation in northern Nevada. Shown are agglomeration 
equipment at center, agglomerated ore stockpile at right, and heaps being leached at left. 



barren solution to maintain the desired 
value. The leaching and washing cycle is 
20 days. Leached residues are trans- 
ported to a tailings disposal area. 

The pregnant solution draining from the 
leaching pad flows by gravity to a reser- 
voir and is pumped through a series of 
five carbon adsorption tanks for recov- 
ery of dissolved gold values. The gold- 
loaded, 12- by 30-mesh, coconut shell ac- 
tivated carbon is transported to the 
millsite for desorption by an alkaline 
alcohol solution. The values desorbed 
from the carbon are electrowon on steel 
wool cathodes, which are refined on site 
to produce dore'' bullion. 

This operation initially tried conven- 
tional heap leaching to process the low- 
grade ore. Conventional heap leaching 
was unsuccessful because of the high clay 



content of the ore, particle segregation, 
and fines migration, which resulted in 
leaching solution channeling (fig. 8). 
Agglomeration pretreatment increased gold 
recovery by 60 pet while decreasing the 
leaching cycle from 50 days to 20 days 
(fig. 9). Even with agglomeration, some 
ore retains approximately 30 pet mois- 
ture, and long washing periods are re- 
quired to recover the dissolved values 
from the agglomerates. The longer wash- 
ing cycle extends the total leaching cy- 
cle to 30 days for some heaps. 

EASTERN NEVADA GOLD HEAP 
LEACHING, 3,000 TPD 

The largest of the operations surveyed 
is located in eastern Nevada and pro- 
cesses approximately 3,000 tpd of ore {1^ 
8^) (fig. 10). The ore is a silicified 
siltstone containing finely disseminated 



11 




FIGURE 8. - Heap leaching gold ore in northern Nevada before agglomeration pretreatment was 
adopted. Notice the ponding on the heaps and the low rate of solution application. 




FIGURE 9. - Agglomerated gold ore being heap leached in northern Nevada. The solution applica- 
tion rate is greatly increased and no ponding is observed. 



12 




FIGURE 10. = Overall view of the eastern Nevada agglomeration-heap leaching operation showing 
agglomeration equipment building, crushed ore stockpile in front center, and heaps in different stages 
of construction in the background. 




FIGURE 11- Drum agglomerator (9 by 21 ft) used in the eastern Nevada operation. 



13 



I 



free gold. Gold-bearing carbonaceous ore 
occurs at depth in the deposit. The 
average grade of the deposit is 0.10 tr 
oz gold per ton. The ore is mined by 
open pit methods and is trucked a short 
distance to the crushing circuit. 

The run-of-mine ore is crushed to a 
nominal 3/4 in by a primary jaw crusher, 
a standard cone crusher, and a short-head 
cone crusher. The crushed ore is con- 
veyed into a building where it is 
agglomerated. 

Two types of ore are processed. Some 
of the oxidized ore contains no clayey 
material, and crushing it generates very 
few fines. Only 6 pet of the material is 
minus 200 mesh ( 4_) . Consequently, it 
does not require a cement binder for ag- 
glomeration. This portion of the ore is 
agglomerated with cyanide solution to 
start the leaching process during heap 
building. 

The second type of crushed ore is mixed 
with 4 to 10 lb of Portland cement (type 
II) per ton of feed in the first portion 
of a 9- by 21-ft rotating drum agglom- 
erator (fig. 11). Cyanide solution is 



sprayed onto the mixture in the middle 
portion of the drum and gives a final 
moisture content of approximately 8 pet. 
The mixture agglomerates in the last por- 
tion of the drum. The design capacity of 
the drum agglomerator is 215 tons of dry 
ore per hour, but it has been operated at 
450 tph. 

The agglomerated ore is conveyed to a 
stockpile where it is loaded by front-end 
loader into trucks for haulage to the im- 
pervious leaching pad. The agglomerates 
cure during the heap building process. 

The permanent heaps are built by driv- 
ing the truck onto the heap and dumping 
the feed over the edge. A tracked dozer 
levels the heap , and the surface is 
ripped before leaching. This heap build- 
ing procedure is not recommended for 
agglomeration-heap leaching of ores that 
are susceptible to compaction (4^). Each 
heap contains approximately 120,000 tons 
of agglomerated ore stacked 15 ft high 
(fig. 12). 

The heaps are leached by sprinkling a 
pH 11.5 solution containing 0.5 lb NaCN 
per ton of solution at a rate of 0.003 to 




FIGURE 12. - Agglomerated heaps being leached in the eastern Nevada gold heap leaching operation. 



14 



0.005 gpm/ft^. The total leaching and 
washing cycle is from 20 to 80 days and 
depends on the ore being processed. The 
pregnant solution collects on the imper- 
vious pads and drains to plastic-lined 
ditches. The solution flows by gravity 
to a 7-million-gal pregnant solution 
pond. 

Pregnant solution from the pond is 
pumped through a series of five activated 
carbon tanks at a rate of 1,000 gpm. The 
barren solution exiting the fifth carbon 
tank, is pumped to the barren solution 
pond for reagent makeup and recycle to 
the heap. The loaded carbon from the 
lead tank is stripped with a caustic cya- 
nide solution at 120° C and 35 psig pres- 
sure. The gold-bearing solution is 
cooled to 85° C before electrowinning. 
The solution is pumped through two 



rectangular electrowinning cells where 
the gold is deposited on steel wool cath- 
odes. The cathodes are refined on-site 
to produce dore bullion containing 92 pet 
gold and 6 pet silver. The dore bars 
are shipped to a custom refinery for 
processing. 

Recoveries by agglomeration-heap leach- 
ing range from 70 to 90 pet, and depend 
on the ore being processed. Conventional 
heap leaching was used before the agglom- 
erating equipment was on-line. Gold re- 
coveries were less than 50 pet because of 
fines migration and the high clay con- 
tent , which caused blinding and channel- 
ing of leaching solution. Agglomeration 
technology has helped maintain targeted 
recoveries in shorter leaching periods 
for heaps with high clay content. 



SUMMARY AND CONCLUSIONS 



The precious metals industry has rapid- 
ly adopted agglomeration-heap leaching 
technology. The five operations de- 
scribed in this report demonstrate the 
versatility and applicability of the 
technology to heap leachable ore types 
for different size operations. A summary 
of agglomerating conditions for the oper- 
ations is shown in table 1. In the West- 
ern United States, there are 36 commer- 
cial operations using agglomeration-heap 
leaching technology. 

For the operations described, agglomer- 
ation pretreatment technology effectively 
improved heap permeability and permitted 
better contact of the cyanide leaching 
solution with the precious metal. As a 
result, percolation rates increased, 
leaching periods decreased, and precious 
metal recoveries were improved over those 
of conventional heap leaching technology. 
Agglomeration pretreatment provides cer- 
tain other advantages. Since the heaps 



are more permeable and drain rapidly, 
soluble losses are minimized and inter- 
mittent sprinkling is permitted. This is 
important in arid climates because solu- 
tion loss by evaporation is minimized by 
spraying the heaps during night hours 
when winds and ambient temperature are 
low. During the period that the heap is 
not being sprayed, the residual cyanide 
in the heap continues to dissolve pre- 
cious metal and enriches the pregnant 
solution when spraying is resumed. The 
greater permeability of the heaps permits 
better washing of dissolved values from 
the heap and decreases the residual cya- 
nide content of the washed, spent heap. 

Even though more reagents and equipment 
are required for agglomeration-heap 
leaching than for conventional heap 
leaching, it is cost effective because of 
the decrease in leaching time and im- 
proved precious metal recoveries. 



15 



TABLE 1. - Summary of agglomerating conditions of the five commercial 
operations described 





Process- 
ing rate, 
tpd 


Feed 

size, 

in 


Binder 


added 


Agglomeration 


Curing 
time, 
hr 


Leach- 
ing 

cycle, 
days 


Recov- 


Operation 


to ore 


Equip- 
ment 
used 


Mois- 
ture, 
pet 


ery, 




Type 


lb/ton 


pet 


Arizona 


20 


3/8 


Cement . 


10 


Drum. . . 


10 


48 


5 


90 


gold heap 
leaching. 




















Colorado 


1,500 


1/2 


. .do ... 


3 


. .do. . . 


13 


8 


3 


90 


gold vat 




















leaching. 






Fly ash 


7 












Arizona 


2,000 


1/2 


Lime ... 


7 


Reverse 


10-12 


72 


7 


I90 


silver heap 
leaching. 










belt. 










Northern 


2,500 


5/8 


Cement . 


7-10 


None^ . . 


9-13 


48-72 


20 


(^) 


Nevada 




















gold heap 
leaching. 




















Eastern 


3,000 


3/4 


. .do. . . 


4-10 


Drum . . . 


8 


(M 


20-80 


70-90 


Nevada 




















gold heap 
leaching. 





















^90 pet of the leachable silver is recovered. 

^Agglomeration occurs as the wetted feed cascades dovm the conical stockpile. A 
drum agglomerator is planned for this operation. 

^Percent recovery is proprietary company information. Agglomeration increased the 
recovery by 60 pet. 

^While heap is built. 



16 



REFERENCES 



1. Heinen, H. J., G. E. McClelland, 
and R. E. Lindstrom. Enhancing Percola- 
tion Rates in Heap Leaching of Gold- 
Silver Ores. BuMines RI 8388, 1979, 20 
pp. 

2. McClelland, G. E., and J. A. 
Eisele. Improvements in Heap Leaching To 
Recover Silver and Gold From Low-Grade 
Resources. BuMines RI 8612, 1982, 26 
pp. 

3. Johanson, J. R. Particle Segrega- 
tion ... and What To Do About It. Chem. 
Eng., (N.Y.), V. 85, No. 11, May 1978, 
pp. 183-189. 

4. Chamber lin, P. D. Heap Leaching 
and Pilot Testing of Gold and Silver 
Ores. Min. Cong. J., v. 67, No. 4, Apr. 
1981, pp. 47-52. 

5. Lewis, A. Producing Gold for $160/ 
Tr Oz in Victor, Colorado. Eng. and Min. 



J., v. 183, No. 10, Oct. 1982, pp. 102- 
105. 

6. Engineering and Mining Journal. 
Pelletizing Aids Tombstone Leaching Oper- 
ation. V. 182, No. 1, Jan. 1981, pp. 94- 
95. 

7. . This Month in Mining. Al- 
ligator Ridge Uses Heap Leaching To Pro- 
duce Gold Bullion Bars. V. 182, No. 8, 
Aug. 1981, pp. 35-37. 

8. McQuiston, F. W. , Jr., and R. S. 
Shoemaker. Heap Leaching Gold and Silver 
Ores. Ch. in Gold and Silver Cyanidation 
Plant Practice. American Institute of 
Mining, Metallurgical, and Petroleum En- 
gineers, Inc., New York, v. 2, 1981, pp. 
13-18. 



*U.S. GOVERNMENT PRINTING OFFICE: 1983-605-015/61 



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